1. Field of the Invention
This invention relates to a system and method for separating nitrogen from methane and other components from natural gas streams. The invention also relates to a system and method for integrating natural gas liquids (NGL) extraction with nitrogen removal. The invention also relates to a system and method for removing excess hydrocarbons from a nitrogen vent stream and optionally recovering helium. The system and method of the invention are particularly suitable for use in recovering and processing feed streams typically in excess of 50 MMSCFD and up to 300 MMSCFD, depending on the concentration of nitrogen in the feed stream.
2. Description of Related Art
Nitrogen contamination is a frequently encountered problem in the production of natural gas from underground reservoirs. The nitrogen may be naturally occurring or may have been injected into the reservoir as part of an enhanced recovery operation. Transporting pipelines typically do not accept natural gas containing more than 4 mole percent inerts, such as nitrogen. As a result, the natural gas feed stream is generally processed to remove such inerts for sale and transportation of the processed natural gas.
One method for removing nitrogen from natural gas is to process the nitrogen and methane containing stream through a Nitrogen Rejection Unit or NRU. The NRU may be comprised of two cryogenic fractionating columns, such as that described in U.S. Pat. Nos. 4,451,275 and 4,609,390. These two column systems have the advantage of achieving high nitrogen purity in the nitrogen vent stream, but require higher capital expenditures for additional plant equipment, including the second column, and may require higher operating expenditures for refrigeration horsepower and for compression horsepower for the resulting methane stream.
The NRU may also be comprised of a single fractionating column, such as that described in U.S. Pat. Nos. 5,141,544, 5,257,505, and 5,375,422. These single column systems have the advantage of reduced capital expenditures on equipment, including elimination of the second column, and reduced operating expenditures because no external refrigeration equipment is necessary. In addition to capital and operating expenditures, many prior NRU systems have limitations associated with processing NRU feed streams containing high concentrations of carbon dioxide. Nitrogen rejection processes involve cryogenic temperatures, which may result in carbon dioxide freezing in certain nodes of the process causing blockage of process flow and process disruption. Carbon dioxide is typically removed by conventional methods from the NRU feed stream, to a maximum of approximately 35 parts per million (ppm) carbon dioxide, to avoid these issues.
Another example is found in UK Patent Application GB 2,208,699. The '699 application cools the feed stream by cross-exchange with the bottom liquid in the second column to provide part of the second column reboiler duty prior to feeding the top of the first column, effectively providing reflux to the first column. The '699 application also links the first column and second column by cooling the overhead stream from the first column by cross-exchange with bottom liquid in the second column to provide part of the reboiler duty for the second column. This concept is commonly referred to as a “heat pump” configuration. Since there is by definition a match of heat requirements or duties for the two different applications, variations in each duty requirement is limited. This limitation translates into a limited range of inlet nitrogen permissible into this style of NRU of approximately a 6-12% range on either side of the design point. If the incoming nitrogen content is outside of the design range then one of the connected heat exchangers has an insufficient amount of heat and the other connected exchanger is short of the required duty requirement. The result is either the amount of nitrogen remaining in the sales gas stream is too high to meet specifications or the amount of methane vented into the atmosphere with the rejected nitrogen is higher than the desired amount, resulting in excess green house gas emissions. In addition having a colder feed to the first column reduces the CO2 tolerance for the systems, as stated in the '699 patent that CO2 must be removed prior to processing. Having a warmer feed to the second column increases the duty required for the reflux in the second tower. Additionally, by linking the first column streams to the reboiler duty of the second column, and since the system of the '699 patent is not as tolerant of wide variations in nitrogen content in the feed stream, the '699 patent would have much higher power requirements for increased nitrogen levels. The '699 patent, like many prior art systems, also links the duties of the second column condenser and reboiler by using an open heat pump cycle where a portion of the bottoms liquid stream is used to provide the reflux duty to several intermediate condensers and an overhead condenser within the second column. Linking these duties decreases costs of the column, but also significantly decreases flexibility in handling higher nitrogen concentrations than original system design.